The complexity of MIB circuits and,
for copper systems, the newness of nitrogen atmosphere processing technology,
affects each of the elements of paste management. The relationship between
paste manufacturers and MIB manufacturers is not as well established as, for
example, that between paste manufacturers and manufacturers of capacitors,
resistor networks, or automotive and television products. At this time, the
small segment of the thick film market occupied by complex MIBs generates only
marginal interest for the paste manufacturers. This, plus the fact that the
circuit must meet military reliability requirements, highlights the need for
the MIB manufacturer to more closely monitor the quality of the paste
manufacturer. Consequently, pastes must be received in a more guarded manner
than conventional pastes. Delay of inspection at the receiving dock will
compromise the materials.

The paste manufacturer must respond
to a more detailed list of paste specifications and documents to preclude
shipment of unacceptable paste materials. It is not uncommon for paste
acceptance, paste qualification, circuit manufacturing, and testing to deplete
more than one-half the stated shelf life of the paste.

Paste manufacturer selection is
critical to program success. The selection process should
be conducted carefully because, in general, the resultant relationship
represents a long-term commitment. Figure 4-2 shows the major steps in paste
selection and evaluation.

Discussions with paste manufacturer
candidates should include as much specific information as possible about their
material for the product to be built, including:

• Review of the paste
manufacturer's identification sheet.

• Inspection of data sheets from
the processed raw materials to the finished product

• A complete set of material
procurement specifications, including paste and coupon or specimen
evaluations

• The nature of the circuit and the
processing conditions used in MIB manufacturing. This should include size,
number of layers, number of firings, etc.

• Methods and equipment used to
measure the pastes both by the paste manufacturer and the MIB
manufacturer

• Agreement on what documentation
will be supplied, i.e., certificate of compliance, qualification tests on a
periodic basis, what data will be supplied with each lot

In addition to material data,
general paste manufacturer-unique information topics should be reviewed,
including:

• Company background

• Location of R&D and
manufacturing

• Business in copper and noble
metals

• Quality assurance
program

• Experience with the type of
system being contemplated

• Traceability of paste lots to raw
materials

• Documentation on each
lot

• Yield of product and how much
rework is acceptable

• Standard test patterns and
standard testing done to each lot

• Recommended processing conditions
including process parameter tolerances and expected product characteristic
changes with out-of-tolerance processing. This is particularly important for
copper thick film systems.

• Critical control parameters in
processing

Table 4-5 lists the leading paste
manufacturers who presently supply copper- based and noble metal systems. The
table represents a snapshot of the manufacturers' product lines and level of
maturity at the time of writing. The materials systems for noble-metal MIBs
are well established and available from several manufacturers. However,
although many companies offer copper thick film systems, almost all Navy
equipments presently using copper thick film MIBs use a single thick film
paste source. Providing adequate communication between manufacturer and
customer is critical for several reasons: the MIB product is uniquely complex;
the product must meet detailed and specific military specifications; and
manufacturer interest, motivation, and ability to satisfy this relatively
small market segment may be marginal.

The prospective manufacturer should
be specifically requested to supply a certificate of compliance, needed label
designations, etc. The customer should provide the manufacturer with a
complete set of material procurement specifications, including paste and
coupon or specimen evaluations. The manufacturer should also be informed of
the nature of the circuit and the processing conditions used in its
manufacture. Independently measured values of paste viscosity, solids content,
and fineness of grind taken by the paste manufacturer and MIB manufacturer,
using common methods, should agree if proper care is taken and similar
equipment is used at each location. The manufacturer must also support the
paste delivery with documentation certifying that the paste conforms to the
specifications of the order.

Incoming materials should be held
in the receiving department until each batch has been fully tested and found
to be acceptable. Alternatively, material could be evaluated as a lot sample
before production quantities are shipped.

The paste manufacturing process is
a lengthy one. Since a use test by a MIB manufacturer on a paste lot may find
the lot unacceptable, paste must be ordered well in advance of anticipated
need (12-16 weeks) to avoid a delay in replacing the lot and performing
another use test. In addition, the guaranteed shelf life of paste is typically
6 months. With so much time required for shipping, receiving, and
qualification, a paste lot may not be fully consumed prior to the shelf life
limits. This requires requalification of the remaining paste before use.

Quality assurance activities for
thick film paste materials are divided into two portions: qualification of a
material at the start of procurement (a part of the material selection
procedure); and acceptance tests performed on a lot basis. Each of these
activities has three elements: paste manufacturer characteristic testing, MIB
manufacturer characteristic testing, and use testing.

Generally, qualification tests and lot acceptance tests
are similar, with qualification requiring larger sample sizes and some
additional tests or evaluations. Table 4-6
identifies what
tests are typically performed for each category.Characteristic tests
consist of:

1Qualification testing is performed initially on all
of the above parmeters after material selection
and prior to release of materials to
production.2Standard test available in Documents of
IPC, ISHM, & Navy Specification
6228507 (See Appendix
B )3Noble metal system dielectrics have a higher
dielectric constant (K = 9-12). Copper system
dielectrics have lower dielectric constant (K= 5 to 7)4Noble
metal system dielectrics are high density/low porosity glasses and do not
require an overglaze. Copper
system dielectrics are more porous and require and
overglaze.

It is expected that these are minimum tests that would be performed by the paste manufacturer as part of quality control and by the MIB manufacturer as part of incoming inspection. Not commonly performed at present, but highly recommended, are thermal gravimetric analysis (TGA) and differential thermal analysis (DTA).

Viscosity. The viscosity characteristics of nitrogen-fired
copper or dielectric pastes may not be like their air-fired counterparts
because of the difference in density of copper versus noble metal and the
difference in organic vehicle appropriate for nitrogen-atmosphere processing.
For example, a great deal of effort has been put into special-viscosity
fine-line gold for use in wire bond interconnecting to silicon IC chips. This
resolution is not needed for MIBs having 5-, 8-, or 10-mil-wide lines.